Decode circuit for code division multiple access receiver

ABSTRACT

To improve a capacity of coherent detection of the spread spectrum (SS) signal, when interference and noise can not be neglected. Amplitude phase estimation unit outputs transmission line estimation value including fluctuations in the transmission lines, using known pilot signals of which transmission sequences are known. The pilot signal is extracted from the despread signal through despreading filter. Reliability measurement unit pursues a reliability value of transmission line estimation value. Interpolation unit generates vectors for compensating the phase, by deciding a method of interpolation on the basis of the reliability. Coherent detection unit compensates the amplitude and phase of information symbol to execute the coherent detection.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to a decoding circuit for receivers inmobile communication systems using code division multiple access (CDMA).

2. Description of the Prior Art

Conventional transmitters for spread spectrum (SS) communication underthe CDMA modulate communication data are modulated beforehand bymodulation techniques. In other words, the modulated communication dataare further modulated by Pseudo Noise (PN) sequence for SS modulation inorder to widen the transmission bandwidth. The widened transmissionbandwidth is used for a plurality of channels, wherein different PNsequences are assigned for every user or every kind of information.

Therefore, receivers firstly despread the wide-band SS signal into itsoriginal bandwidth and then decodes the despread signal by usingconventional techniques. Here, a correlation of PN sequence is detectedbetween the received signal and the receiver side by despread filtersuch as a matched filter which handles the PN sequence as tapcoefficients, or a adaptive filter which optimizes the despreadingprocess by varying the tap coefficients.

The receiving side can pick up a communication signal among multiplexedcommunication signals by detecting the correlation. The picked-up signalis a complete auto-correlation of the desired signal, when the PNsequence of the receiving side has no cross-correlation with other PNsequences. However, in general, PN sequences have cross-correlation witheach other. Therefore, the output from the despread filter containsinterference signals which are not desired by the receiving side. Theabove-mentioned interference signals increase with the number ofcommunication channels and degrade quality of received signal which isfurther degraded by thermal noise in the receiver and noises intransmission lines.

In the CDMA transmission, wherein information signal is spread by PNsignal of which speed is higher than that of the information signal,prescribed pilot signals are inserted periodically in between theinformation signals, as shown in FIG. 8. Therefore, phase vectors of theinformation symbols are obtained by an interpolation on the basis ofphase vectors of the pilot symbols, as shown in FIG. 9. Here, phasevector 1001 shows a track of end point of every symbol due tofluctuations in the transmission lines. Phase vector 1002 is a phasevector of every information symbol obtained by the interpolation. Phasevector 1003 is a phase vector which is an average phase vector in pilotsymbol periods.

A decode circuit implementing the above-mentioned method is shown inFIG. 10. The decode circuit as shown in FIG. 10 comprises matched filter803 which operates under chip cycle Tc with a tap length of 1 symbol,interpolation detection unit 807, and Decision unit 109.

The decode circuit as shown in FIG. 10 despreads SS signal 101, executescoherent detection, and outputs identified signal 102 on the basis ofcoherent detection.

Matched filter 803 comprises a plurality of delay units 804, a pluralityof tap coefficient multipliers 805, and tap signal adder 806. Further,interpolation synchronous detector 807 for compensating a phasedifference from matched filter 803 comprises amplitude phase estimationunit 105, first order interpolation unit 809, and coherent detectionunit 108.

Amplitude phase estimation unit 105 pursues transmission estimationvalue 301 concerning the amplitude and phase in the transmission line,on the basis of a received phase of the pilot signal contained in thedespread signal from matched filter 806.

First order interpolation unit 809 interpolates a received phase vector,inspecting a location of each information symbol.

Coherent detection unit 108 executes coherent detection, by compensatingerrors in the amplitude and phase of the information symbol on the basisof the received phase vector which is obtained by first orderinterpolation unit 809.

Decision unit 109 identifies a phase of the output from interpolationcoherent detection unit 807. The output from Decision unit 109 isidentified signal 102.

SS signal 101 as shown in FIG. 10 which contains known pilot symbol isinputted into matched filter 803 which has “M” taps extracted every chipcycle Tc. Here, an integer “M” is a spread factor. Signal vectors ofeach tap inputted into matched filter 803 are multiplied by tapcoefficient vectors of which coefficient is determined by PN sequence.The tap coeflicient vector is C_(M), C_(M−1), . . . , C₂, Cl, as shownin FIG. 10. Then, the signals from each tap are added in tap signaladder 806 and outputted as a despread signal. Then, amplitude phaseestimation unit 105 in interpolation coherent detection unit 807pursues, by using the known pilot symbol, a received phase vector whichrepresents errors of amplitude and phase due to the fluctuation oftransmission lines. First order interpolation unit 809 interpolates areceived phase vector, inspecting a location of each information symbol.Coherent detection unit 108 executes coherent detection, by compensatingerrors in the amplitude and phase of the information symbol on the basisof the received phase vector which is obtained by first orderinterpolation unit 809. Decision unit 109 identifies a phase of theoutput from interpolation coherent detection unit 807. The output fromDecision unit 109 is identified signal 102.

The conventional decode circuit as shown in FIG. 10 compensates fadingdistortion by using the pilot signal.

However, when received SS signal contains the interference and noisewhich can not be neglected, the pilot signal is also distorted.Therefore, the coherent detection is not correctly executed, because thetransmission lines can not be estimated correctly.

Therefore, the conventional decode circuit has a disadvantage that thecoherent detection is not correctly executed, when the interference andnoise can not be neglected.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a decodecircuit with improved capability of coherent detection such thattransmission lines are estimated correctly, even when the received SSsignal contains much interference and noise.

A decode circuit of the present invention comprises a despreading filterfor despreading spread spectrum (SS) signal, an amplitude phaseestimation unit which estimates the amplitude and phase in transmissionlines on the basis of pilot signals included in the despread signal, areliability measurement unit for calculating a reliability value of theestimated amplitude and phase in the transmission lines, aninterpolation unit for compensating the phase of information symbols bydeciding a method of interpolation on the basis of the reliability, acoherent detection unit which detects the despread signal by using theinterpolated estimation value in the transmission lines, and an decisionunit for identifying the output from the coherent detection unit on thebasis of its phase.

In short, the decode circuit of the present invention measures thereliability of the transmission line estimation value. Further, aninterpolation method is decided on the basis of the reliability. Then,compensation vectors for compensating the phase are generated by thedecided interpolation method to execute the coherent detection.

Therefore, the coherent detection is executed exactly, even when theinterference and noise cannot be neglected.

Here, the reliability measurement unit may also calculate a reliabilityvalue of the detected signal outputted from the coherent detection unit.

According to the decode circuit of the coherent detection of the spreadspectrum (SS) signal is optimized, even when the interference and noisecan not be neglected.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a block diagram of a decode circuit of the first mode ofembodiment of the present invention.

FIG. 2 is an illustration for explaining an optimum interpolation byusing pilot symbols.

FIG. 3 is a block diagram of reliability measurement unit 106 andinterpolation unit 107 as shown in FIG. 1.

FIG. 4 is a block diagram of a decode circuit of the second mode ofembodiment of the present invention.

FIG. 5 is a block diagram of reliability measurement unit 408 as shownin FIG. 4.

FIG. 6 is a block diagram of reliability measurement unit 408 a in adecode circuit of the third mode of embodiment of the present invention.

FIG. 7 is a block diagram of reliability measurement unit 408 b in adecode circuit of the fourth mode of embodiment of the presentinvention.

FIG. 8 is an illustration for explaining a signal flame structureincluding pilot symbols.

FIG. 9 is an illustration for explaining a first order interpolationusing the pilot symbol.

FIG. 10 is a block diagram of a conventional decode circuit for spreadspectrum (SS) signals.

PREFERRED EMBODIMENT OF THE INVENTION

The mode of embodiment of the present invention is explained, referringto the drawings.

First Mode of Embodiment

A block diagram of a decode circuit of the first mode of embodiment ofthe present invention is shown in FIG. 1. Reference numbers are commonlyused in FIGS. 1 and 10.

The decode circuit as shown in FIG. 1 comprises despreading filter 103,interpolation coherent detection unit 104, and Decision unit 109.Matched filter 803 as shown in FIG. 10 is an example of despreadingfilter 103.

Further, interpolation coherent detection unit 104 for compensating theamplitude and phase from despread filter 103 comprises amplitude phaseestimation unit 105, reliability measurement unit 106, interpolationunit 107, and coherent detection unit 108.

SS signal 101 as shown in FIG. 1 which contains known pilot symbolsperiodically is despread by despreading filter 103 and a SS signal in acertain channel is outputted. Then interpolation coherent detection unit104 estimates transmission line estimation value 301 concerningcharacteristics of the amplitude and phase of the transmission line byusing amplitude phase estimation unit 105.

Further, reliability measurement unit 106 calculates a reliability oftransmission line estimation value 301. Interpolation unit 107 decidesan interpolation method on the basis of the calculated reliability, andinterpolates transmission line estimation value 301 on the basis ofpositions of information symbols. For example, the transmissioncharacteristics are estimated, taking into consideration transmissionline estimation value 301 which is obtained in adjacent symbol intervalsas shown in FIG. 9, when the calculated reliability is greater than aprescribed threshold. In this case, estimation errors are considered tobe small. On the other hand, when the calculated reliability is ssmallerthan a prescribed threshold, the transmission characteristics areestimated, without taking into consideration transmission lineestimation value 301, as shown in FIG. 2.

In FIG. 2, received phase vector 1004 indicates average received phasevector averaged over the pilot symbol period after 2 slots. Receivedphase vector 1005 indicates average received phase vector averaged overthe pilot symbol period after 1 slot. Received phase vector 1006indicates interpolated received phase vector. Received phase vector 1007indicates average received phase vector averaged over the present symbolperiod. Here, received phase vector 1007 can not be employed forinterpolation, due to its low reliability.

Then, coherent detection unit 108 executes coherent detection ininterpolation unit 107 on the basis of transmission line estimationvalue interpolated for every information symbol. Decision unit 109determines the phase of the output signal from interpolation coherentdetection unit 104 and outputs the above-mentioned output signal frominterpolation coherent detection unit 104 as an identified signal 102.

Despreading filter 103, amplitude phase estimation unit 105, coherentdetection unit 108, and decision unit 109 are similar to thecorresponding elements as shown in FIG. 10. Therefore, only reliabilitymeasurement unit 106 and interpolation unit 107 are further explainedconcretely, referring to FIG. 3.

As shown in FIG. 3, reliability measurement unit 106 comprises powercalculation unit 303, threshold supply unit 304, and reliabilitygeneration unit 305. Interpolation unit 107 comprises interpolationmethod instruction unit 306, first order interpolation unit 307, andsecond interpolation 308.

Power calculation unit 303 calculates a power of transmission lineestimation value 301 by squaring the amplitude of transmission lineestimation value 301.

Threshold supply unit 304 supplies reliability generation unit 305 witha prescribed threshold.

Reliability generation unit 305 pursues the reliability of transmissionline estimation value 301, by comparing the threshold from thresholdsupply unit 304 with the power from power calculation unit 303.Concretely, reliability is set to be “1”, when the power is greater thanthe threshold, because the reliability is high in this case, whilereliability is set to be “0”, when the power is smaller than thethreshold, because the reliability is low in this case. The reliabilityis sent to interpolation method instruction unit 306.

Interpolation method instruction unit 306 transfers transmission lineestimation value 301 to first order interpolation unit 307, when thereliability is “1”, while it transfers transmission line estimationvalue 301 to second order interpolation unit 308, when the reliabilityis “0”.

First order interpolation unit 307 interpolates the transmission lineestimation value estimated in the adjacent pilot symbol periods intoeach information symbol, as shown in FIG. 9.

Second order interpolation unit 308 interpolates the transmission lineestimation value estimated in four pilot symbol periods into eachinformation symbol, as shown in FIG. 2, neglecting the transmission lineestimation value of which power is small. Here, the four pilot symbolperiods are two periods before the present period and two periods afterthe present periods.

Next, the operation of first mode of embodiment of the present inventionis explained, referring to the drawings.

As shown in FIG. 3, power calculation unit 303 inputs transmission lineestimation value 301 and calculates a power of transmission lineestimation value 301 by squaring the amplitude of transmission lineestimation value 301. Then, reliability generation unit 305 pursues thereliability value, by comparing the threshold from threshold supply unit304 with the power of transmission linr estimation value 301.

Interpolation method instruction unit 306 transfers transmission lineestimation value 301 to first order interpolation unit 307, when thereliability is “1”. First order interpolation unit 307 interpolates thetransmission line estimation value estimated in the adjacent pilotsymbol periods into each information symbol, as shown in FIG. 9. Here,the adjacent pilot symbol periods are a period before the present periodand a period after the present period.

On the contrary, interpolation method instruction unit 306 transferstransmission line estimation value 301 to second order interpolationunit 308, when the reliability is “0”. Second order interpolation unit308 interpolates the transmission line estimation value estimated infour pilot symbol periods into each information symbol, as shown in FIG.2, neglecting the transmission line estimation value of which power issmall. Here, the four pilot symbol periods are two periods before thepresent period and two periods after the present periods.

As shown in FIG. 2, three values among four transmission estimationvalues are employed for the second order interpolation, neglecting oneof the estimation value of which power is small. Zero orderinterpolation, and weighted interpolation may be also employed. Further,higher order interpolation may be employed, using four or moreestimation values.

Not only power itself but also a variance of power may be employed inthe interpolation. In this case, the method as FIG. 9 is employed for asmall variance, while the method as set forth in FIG. 2 is employed fora great variance.

Furthermore, a plurality of thresholds my be introduced, or thethreshold may be varied adaptively.

Second Mode of Embodiment

A block diagram of decode circuit of the second mode of embodiment ofthe present invention is shown in FIG. 4. Reference numbers are commonlyused in FIGS. 1 and 4.

The decode circuit as shown in FIG. 4 comprises despreading filter 103,interpolation coherent detection unit 404, reliability measurement unit408, rake combining unit 409, and decision unit 109.

Further, interpolation coherent detection unit 404 for compensating thephase of the signal outputted from despreading filter 103 comprisestransmission line amplitude phase estimation unit 105, interpolationunit 107, and coherent detection unit 108.

Comparing with the first mode of embodiment as shown in FIG. 1, thedecode circuit of the second mode of embodiment has reliabilitymeasurement unit 408 outside interpolation coherent detection unit 404,while reliability measurement unit 106 is provided inside interpolationcoherent detection unit 104. Further, rake combining unit 409 is added.

Reliability measurement unit 408 measures the reliability of a signaldetected by coherent detection unit 108. Rake conbining unit 409combines a plurality of detected signals. The operation of rakecombining unit 409 is well known.

FIG. 5 is a block diagram of reliability measurement unit 408 as shownin FIG. 4.

Input terminal 501 of reliability measurement unit 408 accepts thesignal detected by coherent detection unit 108. Input terminal 502accepts the output from rake combining unit 412 which combines signalsfrom a plurality of paths. Further, input terminal 503 outputs thereliability value.

Reliability measurement unit 408 comprises SIR (signal to interferenceratio) calculation unit 504, threshold supply unit 505, and reliabilitygeneration unit 506.

SIR calculation unit 504 calculates SIR of the signal which is processedby coherent detection unit 108. SIR may be calculated, for example, byinversely modulating the detected signal by a known sequence which makesup a known pilot symbol. In this case, the SIR is defined as a ratio ofthe squared value of the average of the above-mentioned inverselymodulated signal to the variance of the above-mentioned signal.

Reliability generation unit 506 pursues the reliability value, bycomparing the threshold from threshold supply unit 505 with the SIRvalue. Concretely, reliability is set to be “1”, when the SIR is greaterthan the threshold, because the reliability is high in this case, whilereliability is set to be “0”, when the SIR is smaller than thethreshold, because the reliability is low in this case.

Third Mode of Embodiment

In the third mode of embodiment, reliability measurement unit 408 asshown in FIG. 4 of the second mode of embodiment is replaced byreliability measurement unit 408 a as shown in FIG. 6.

Reliability measurement unit 408 a comprises Decision unit 601, errorrate calculation unit 602, threshold supply unit 505, and reliabilitygeneration unit 506.

Decision unit 601 executes determination and identification of thedetected signal from interpolation coherent detection unit 108 throughinput terminal 501. Error rate calculation unit 602 calculates an errorrate of the Identified signal, by using a part of the Identified signalof which transmission sequence is known. Then, reliability generationunit 506 pursues the reliability value, by comparing the threshold fromthreshold supply unit 505 with the error rate. Concretely, reliabilityis set to be “1”, when the error rate is smaller than the threshold,because the reliability is high in this case, while reliability is setto be “0”, when the error rate is greater than the threshold, becausethe reliability is low in this case.

Fourth Mode of Embodiment

In the fourth mode of embodiment, reliability measurement unit 408 asshown in FIG. 4 of the second mode of embodiment is replaced byreliability measurement unit 408 b as shown in FIG. 7.

Error rate is measured by using a whole block of the Identified signalin the fourth mode of embodiment, although only a part of the Identifiedsignal is used in the third mode of embodiment.

Reliability measurement unit 408 b comprises Decision units 601 and 701,error rate calculation unit 703, threshold supply unit 505, andreliability generation unit 506. Decision units 601 and 701, thresholdsupply unit 505, and reliability generation unit 506 are similar tothose as shown in FIG. 6.

Decision unit 702 executes determination and identification of thedetected signal from rake combining unit 409 through input terminal 502.

Error rate calculation unit 703 calculates an error rate of the detecteddata, on the basis of the outputs from Decision units 601 and 702. Here,the input of Decision units 601 is a signal detected by interpolationcoherent detection unit 108, while the input of Decision units 702 isthe output of rake combining unit 409. Then, reliability measurementunit 506 pursues the reliability value, by comparing the threshold fromthreshold supply unit 505 with the error rate.

Although the present invention has been shown and described with respectto the best mode embodiment thereof, it should be understood by thoseskilled in the art that the foregoing and various other changes,omissions, and additions in the form and detail thereof may be madetherein without departing from the spirit and scope of the presentinvention.

For example, rake combining unit in the first to fourth modes ofinvention may be replaced by diversity combining unit, de-interleaveunit, error correction unit. Further, interpolation may be executed onthe basis of an error rate of pilot symbol of which transmissionsequence is known, or check bit inserted into a flame, by using a rakecombined signal only.

What is claimed is:
 1. A decode circuit for decoding spread spectrumsignal (SS) including a pilot signal, which comprises: a despreadingfilter for despreading said spread spectrum signal; an amplitude phaseestimation unit which estimates the amplitude and phase of the spreadspectrum signal on the basis of said pilot signal; a reliabilitymeasurement unit for calculating reliability of an output of saidamplitude phase estimation unit based upon a comparison of the output toa predetermined threshold; an interpolation unit for compensating thephase of information symbol included in the output of said amplitudephase estimation unit; a coherent detection unit which executes coherentdetection on the basis of the output from said interpolation unit andthe output from said despreading filter; and a decision unit foridentifying the phase of the output signal from said coherent detectionunit.
 2. The decode circuit according to claim 1, wherein saidreliability measurement unit comprises: a power calculation unit forcalculating a power on the basis of the output from said amplitude phaseestimation unit; a threshold supply unit for supplying at least saidpredetermined threshold; and a reliability generation unit forcalculating reliability of the output of said amplitude phase estimationunit, by said comparison of said predetermined threshold with the outputfrom said power calculation unit; wherein said reliability is set to be“1”, when said output from said power calculation unit is greater thansaid predetermined threshold, while said reliability is set to be “0”,when said output from said power calculation unit is smaller than saidpredetermined threshold.
 3. The decode circuit according to claim 1,wherein said interpolation coherent detection unit comprises: a firstorder interpolation unit for interpolating the estimated output fromsaid amplitude phase estimation unit which is estimated on the basis ofthe adjacent two pilot symbols before and after an information symbol; asecond order interpolation unit for interpolating the output from saidamplitude phase estimation unit which is estimated on the basis of theadjacent two pilot symbols before said information symbol and theadjacent two pilot symbols after said information symbol; and aninterpolation method instruction unit for transferring said output fromsaid amplitude phase estimation unit to said first order interpolationunit, when said reliability is “1”, and for transferring said outputfrom said amplitude phase estimation unit to said second orderinterpolation unit, when said reliability is “0”.
 4. A decode circuitfor decoding spread spectrum (SS) signal including a pilot signal, whichcomprises: a despreading filter for despreading said spread spectrumsignal; an amplitude phase estimation unit which estimates the amplitudeand phase of the spread spectrum signal on the basis of said pilotsignal; an interpolation unit for compensating the phase of informationsymbol included in the output of said amplitude phase estimation unit; acoherent detection unit which executes coherent detection on the basisof the output from said interpolation unit and the output from saiddespreading filter; a reliability measurement unit for calculatingreliability of the output of said coherent detection unit based upon acomparison of the output from said coherent detection unit to apredetermined threshold; and a decision unit for identifying the phaseof the output signal from said coherent detection unit.
 5. The decodecircuit according to claim 4, wherein said reliability measurement unitcomprises: a signal to interference ratio (SIR) calculation unit forcalculating an SIR of the output from said coherent detection unit; athreshold supply unit for supplying said predetermined threshold; and areliability generation unit for calculating said reliability, bycomparing said predetermined threshold with said SIR, wherein saidreliability is set to be “1”, when said SIR is greater than saidpredetermined threshold, while said reliability is set to be “0”, whensaid SIR is smaller than said predetermined threshold.
 6. The decodecircuit according to claim 4, wherein said reliability measurement unitcomprises: an error rate calculating unit for calculating an error rateof the output from said decision unit; a threshold supply unit forsupplying predetermined threshold; and a reliability generation unit forcalculating said reliability, by comparing said predetermined thresholdwith said error rate, wherein said reliability is set to be “1”, whensaid error rate is smaller than said predetermined threshold, while saidreliability is set to be “0”, when said error rate is greater than saidpredetermined threshold.
 7. The decode circuit according to claim 4,wherein said reliability measurement unit comprises; another decisionunit for identifying the output from said coherent detection unit, arake combining unit for combining a plurality of detected signals; otherdecision unit for identifying the output from said rake combining unit;an error rate calculating unit for calculating an error rate on thebasis of the output from said another decision unit and the output fromsaid other decision unit; a threshold supply unit for supplying one ormore predetermined threshold; and a reliability generation unit forcalculating said reliability, by comparing said predetermined threshold.8. The decode circuit according to claim 4, wherein said interpolationcoherent detection unit comprises: a first order interpolation unit forinterpolating the estimated output from said amplitude phase estimationunit which is estimated on the basis of the adjacent two pilot symbolsbefore and after an information symbol; a second order interpolationunit for interpolating the output from said amplitude phase estimationunit which is estimated on the basis of the adjacent two pilot symbolsbefore said information symbol and the adjacent two pilot symbols aftersaid information symbol; and an interpolation method instruction unitfor transferring said output from said amplitude phase estimation unitto said first order interpolation unit, when said reliability is “1”,and for transferring said output from said amplitude phase estimationunit to said second order interpolation unit, when said reliability is“0”.
 9. The decode circuit according to claim 1, wherein saiddespreading filter is a matched filter.
 10. The decode circuit accordingto claim 4, wherein said despreading filter is a matched filter.